The present invention relates generally to data transmission systems, and more particularly to an improved method and apparatus for the self-clocking transmission of digital data signals.
Among the self-clocking data transmission techniques in the prior art, one technique commonly referred to as "polar return to zero" utilizes different voltage levels within a bit interval to encode a data signal. For example, a positive voltage level with respect to a reference voltage level indicates a binary one state, while a negative voltage level with respect to the reference voltage level indicates a binary zero state. Another technique commonly referred to as Manchester coding provides a transition during each bit interval, where the direction of the transition determines the binary state of the bit. For example, a positive transition during a bit interval may represent a binary one bit, while a negative transition may represent a binary zero bit.
However, in order to properly receive data signals transmitted according to these prior art techniques, it is essential that timing relationships between successive bits of the data signal be accurately maintained at the data transmitter and properly recognized at the data receiver. Furthermore, proper reception at the data receiver is also dependent on recovery of the clock signal and precise definition of the bit interval. Thus, transmission systems utilizing these prior art techniques are very sensitive to speed and timing variations in the transmission of the data signals, and require that receiving apparatus include expensive and complex circuitry to compensate for such variations.